1. Field of the Invention
The present invention relates to a method for drying a ceramic formed body by hot air.
2. Description of the Related Art
A honeycomb structure of ceramics is widely used for catalyst carriers and various filters. Recently, the honeycomb structure has gotten attention as a filter for trapping particulate matter exhausted from diesel engine (hereinafter referred to as DPF). In addition, a monolithic structure of ceramics is excellent in physical strength, durability, corrosion resistance, etc., and used, for example, for removal of suspended matter, bacteria, dust, or the like, present in liquid or gas in a field of a water treatment, an exhaust-gas treatment, pharmaceuticals or food. A honeycomb structure means a structure having a porous ceramic peripheral wall and through-holes separated from each other by porous ceramic partition walls disposed inside the peripheral wall. A monolithic structure means a structure having a porous ceramic substrate provided with a plurality of through-holes. Besides these structure, ceramic formed bodies each having at least one through-hole including a ceramic formed body of a tube structure with one through-hole have been spreading in various fields.
In general, a ceramic formed body having at least one through-hole is produced by the process of: adding a heat-gelling binder, water and other additives to a ceramic material to form mixed clay; forming a predetermined formed body out of the mixed clay by press, extrusion or the like; drying the formed body to gelate it; and firing it. However, in the above ceramic formed body, there arises a problem of deformation such as a bending deformation or a torsional deformation generated in the ceramic formed body in the drying process or a problem of crack generation in the firing process after drying. In particular, a honeycomb structure among ceramic formed bodies each having at least one through-hole sometimes has a structure in which a plurality of honeycomb structural segments are unitarily joined with each other. In this case, when a bending deformation or a torsional deformation is generated in some of the segments, it is difficult to join them together, and therefore, it is impossible to obtain a ceramic formed body having a precise configuration.
There has conventionally been known a method for drying a ceramic formed body, in which a microwave-drying step and a hot-air-drying step are combined with each other, as a method for rapidly drying a ceramic formed body with suppressing a bending deformation or the like upon drying. For example, JP-A-2001-130970 discloses a method in which a ceramic formed body is disposed so that the hot air can smoothly pass through the through-hole (i.e., the axial direction of the through-hole is parallel to the direction of the hot air) in order that moisture is vaporized as uniformly as possible in the hot-air-drying step.
However, it seems that such a drying method is not necessarily preferable as a means against the problem of deformation during drying or crack generation during firing though the method promotes drying inside the through-hole(s) by passing the hot air through the through-hole(s).